The present invention relates to a focus detecting device for use in an optical device, such as a camera, a slide projector and the like.
The practical realization of a focus detecting device or an automatic focus control device has been earnestly sought because it would not only eliminate the differences in the focus control operations as carried out by different individuals, but would also provide for simplified operation of optical devices. Although several such devices have been proposed, most have not yet reached the stage of experimentation because they have not taken into account practical problems which often arise. Especially for cameras, it is important to have reduced size and weight as well as reduced consumption of electric power. An additional indispensable condition for a focus detecting system used in a camera is that it be free from any influence of blur due to handling and flickering in the brightness of an object. Unfortunately, optical information detected by a photoelectric element of a focus detecting device is known to include, not only the necessary information for focus control, but also noise resulting from handling blur and the brightness flickering.
In the invention of Japanese Patent Publication No. 487,934, for instance, the detection is carried out by the use of a DC bridge circuit, although an optical system similar to that of the present invention is employed. As a result, the focus detection becomes unstable since it is subject to influences from the handling blur, dispersion and characteristic inconsistency of the photoelectric element, and other noise. Moreover, the published invention is disadvantageous in that the electric processing of its detected signal is difficult since the signal is of d.c. characteristic. These disadvantages form a major cause for preventing the focus detecting device from being put into practice, and the device cannot exceed the stage of experimentation.